1. Field of the Invention
The present invention relates to a sheet curl correction apparatus for correcting a curl produced in a sheet such as paper on which an image is formed in an image forming apparatus such as a copying machine, a printer, a facsimile, and the like, and to an image forming apparatus including the sheet curl correction apparatus.
2. Related Art
In an image forming apparatus such as a copying machine, a toner image is formed on a sheet, such as paper, in an image forming unit such as a photoreceptor drum and the like, and then the toner image is fixed to the sheet by application of heat and pressure in a fixing part. Consequently, a curl tends to be produced in the sheet after passing through the fixing part. When a sheet with this type of curl is discharged as it is, a space is produced between a plurality of sheets due to the curl when the sheets are stacked in a discharging tray. This phenomenon poses problems in relation to sheet handling such as an adverse effect on storage performance or the sheets becoming disarrayed.
A sheet curl correction apparatus has been applied in which the curl in the sheet is corrected by passing the sheet between a pair of curl correction rollers.
In this connection, the curl in a sheet includes “upward curl” in which a distal end of the sheet is bent towards an upper surface of the sheet and “downward curl” in which the distal end of the sheet is bent towards a lower surface of the sheet. There is a need for a curl correction apparatus that corrects both upward curl and downward curl.
To respond to this need, a curl correction apparatus has been proposed in which the orientation of the pair of curl correction rollers is interchanged by rotating a curl correction unit including a supporting member for rotatably supporting the pair of curl correction rollers, such that a direction of curl correction applied by the pair of curl correction rollers is switchable.
Furthermore, a curl correction apparatus has been proposed that includes a curl correction unit that has a supporting member for rotatably supporting three curl correction rollers, and a detection means that is disposed upstream of the curl correction unit in a direction of sheet feeding and detects a direction of curl in the fed sheet (upward curl or downward curl). The curl correction apparatus controls the curl correction unit to rotate based on the detection of the curl performed by the curl detection means such that the three curl correction rollers can vary directions of curl correction.
However, there is a demand for further downsizing of the curl correction apparatus.
In addition, this type of image forming apparatus does not simply print an image only on one side of a sheet of paper, and includes a duplex printing function that enables printing on both sides. A duplex printing function is realized by inverting a sheet of paper that has been printed on one side without discharging, and then printing on the other side.
This type of image forming apparatus may be incapable of feeding paper by a discharging roller or the like when a curl is produced in a sheet of paper inside the image forming apparatus, and a paper jam may be caused. Consequently, some image forming apparatus includes a curl correction device to remove a curl produced in a sheet of paper.
In addition, there is a technique of preventing a curl produced in a sheet of paper without using a curl correction apparatus. More specifically, a contact point of a pressure roller lying on a common tangent line between a fixing roller and a downstream roller is disposed relative to the fixing roller more downstream of a pressure contact part than a line connecting rotational centers of the fixing roller and the pressure roller. Accordingly, the technique adjusts a shape of the pressure part so as to regulate the curl in the sheet of paper.
This technique allows not only formation of at least two nips in mutually different directions in the pressure contact part, but also adjustment of a length of a nip that lies downstream in a direction of feeding the sheet of paper. Accordingly, it may be possible to regulate the curl in the sheet of paper without a curl correction apparatus, or the like.
The apparatuses used in the conventional techniques described above have an object of suppressing a curl in a sheet of paper without a curl correction apparatus, or the like. However, since there is a situation in which a curl occurs depending on a type of paper, a type of toner, or a method of duplex printing, a curl correction apparatus is required to improve the printing quality.
Some copying apparatus often uses sheets of cut paper that are cut to a predetermined size instead of a roll of paper. Consequently, although a sheet of paper does not curl at the commencement of the image forming process, curling may occur when the sheet of paper passes through the fixing apparatus. When the image forming apparatus is a copying apparatus, for example, an image is formed on a sheet of paper by fixing a toner image with a fixing apparatus after an image transfer apparatus transfers the toner image to the sheet of paper. Generally, the fixing apparatus is composed of a heat roller and a pressure roller, and the toner is fused and fixed to the sheet of paper by application of heat and pressure to the toner. In this manner, the sheet of paper tends to curl in the fixing apparatus due to heating and pressing of the sheet of paper, and a curl correction apparatus may be required downstream of the fixing apparatus to realize high quality printing.
In this connection, there is a technique in which a curl correction apparatus that decurls (corrects a curl) duplex-printed paper enables the direction of decurling to be switched (for example, a rotary decurling apparatus). However, current techniques do not enable a sufficient time for switching the decurl direction after image data stored in a video buffer (a buffer for retracting image data temporarily before image forming) is sent from the video buffer to an engine control unit (after starting image extraction).
Although calculation of a coverage rate of a sheet of paper is adopted to determine the decurl direction, it is only after image data is sent to the engine control unit that the coverage rate is acquired. Consequently, when multiple print runs are performed, the first copy is outputted as a sample, and then the curl correction apparatus is controlled during the second and subsequent runs.
Therefore, a problem arises in that a correct decurl process is not executed in the first print run when the multiple print runs are performed and the printing quality of the first print run cannot be improved.